A sealing apparatus for a filter changing device is disclosed in German Patent Specification No. 1 919 269. In such arrangement, a pressure ring, which is acted upon by hydraulic piston and cylinder arrangements, is moved towards the displaceable slide member in order to effect sealing thereof in opposition to the pressure exerted by the extruded material.
In the event of a changeover of the filters disposed in the slide member, the hydraulic cylinders are initially relieved of pressure. The slide member can then be hydraulically displaced, into the changeover position, at right angles to the flow direction of the material. Simultaneously, the second filter disposed in the slide member is displaced into the path of the material in the flow channel. Accordingly a new, clean filter can be disposed in the material flow channel within a very short period of time.
The frequency with which the filter needs to be changed depends upon the degree of pollution in the extruded material. Each time that the filter is changed, the pressure must be removed from that filter slide member and, after the filter change, a fresh build-up of pressure must be achieved.
The hydraulic cylinders which are used for producing the pressure build-up are interconnected by means of a ring conduit or circuit. This circuit utilises a large number of component parts and, in addition, occupies a large volume around the extrusion head. If one portion of the extrusion head is upwardly pivotable, this spatial requirement for the ring conduit is, generally, not available.